The human eye is a complex sensory organ. It includes a cornea, or clear outer tissue which refracts light rays en route to the pupil, an iris which controls the size of the pupil thus regulating the amount of light entering the eye, and a lens which focuses the incoming light through the vitreous to the retina. The lens is often considered to have 3 portions, namely a nucleus, a cortex around the nucleus and an outer capsular region. In the adult lens, the nucleus tends to be harder and has a relatively constant sectional refractive index, whereas the refractive index of the cortex is known to exhibit a gradient. Any obstruction or loss in clarity within these structures causes scattering or absorption of light rays resulting in diminished vision.
With age, there is a loss of lens flexibility and/or lens transparency. The natural flexibility of the lens is essential for focusing light onto the retina by a process referred to as accommodation. Accommodation is the process by which the eye adjusts its focus for visual objects at different distances. A common condition known as presbyopia results from diminished flexibility of the lens thus reducing near vision acuity. Presbyopia usually begins to occur in adults during their mid-forties; conventionally, these near vision problems are alleviated with glasses or contact lenses.
Another cause of diminished vision is cataracts which is associated with a loss of lens transparency in the aging eye. Some treatments involve the surgical removal of the natural lens. An artificial lens is then needed to restore vision. Three types of prosthetic lenses are available: cataract glasses, external contact lenses, and IOLs. Cataract glasses have thick lenses, are uncomfortably heavy, and cause vision artifacts such as central image magnification and side vision distortion. Contact lenses resolve many of the problems associated with glasses, but require cleaning, are difficult to handle (especially for elderly patients with symptoms of arthritis), and are not suited for persons who have restricted tear production. More particularly, contact lens for restoring vision after lens removal (called “aphakia”) are necessarily very thick due to the high power required. Such thick contact lenses are uncomfortable and cannot deliver sufficient oxygen to support long-term ocular health. IOLs are used in the majority of cases to overcome the aforementioned difficulties associated with cataract glasses and contact lenses.
Known IOLs include non-deformable, foldable and expansible lenses, which may be formed from materials such as acrylics, hydrogels or polysiloxanes. These IOLs are implanted by making an incision in the cornea and inserting a preformed IOL. To treat cataracts, the natural lens is removed before the IOL is implanted. In some procedures, the capsule is left in place following lens extraction. The IOL is then implanted inside the capsule via the capsulorhexis; a hole typically of a few mm in diameter made at the anterior capsule surface. The capsulorhexis is made to provide an opening from which the content (cortex and nucleus) of the lens can be removed during the procedure. To minimise trauma during implantation, foldable and expansible IOLs have been developed. These lenses may be rolled up and inserted through a small tube, which allows a smaller incision to be made in the cornea as well as a smaller diameter capsulorhexis to be made in the capsule. Smaller incisions and rhexes provide quicker post-op recovery as well as improved post-op visual outcomes due to less likelihood of distortion of the cornea. For example, dehydrated hydrogels can be used with these small incision techniques. Hydrogel lenses are dehydrated before insertion and naturally rehydrate once inside the capsular bag. To be suitable as IOLs, these deformable lenses require not just appropriate optical properties, but also mechanical properties, such as structural integrity and elasticity, to permit them to deform during implantation and then regain their shape in vivo. However, currently available IOLs are still relatively much more rigid than the young, flexible natural lens. Thus, such IOLs are not capable of accommodating when in vivo, and so are not an optimal solution as they do not restore the near vision capability (accommodation) of the natural young eye.
To further develop IOLs and reduce surgical incisions to below 1.5 mm, techniques utilising injectable IOLs have been suggested. Injectable IOLs would be implanted by lens filling (or refilling) procedures, such as Phaco-Ersatz. In such a procedure the natural crystalline material of the lens is extracted while the lens capsule-zonule-ciliary body framework is maintained. The intact lens capsule is then refilled by injecting a low viscosity material into the empty capsular bag through a small diameter capsulorhexis. The material may then be cured in situ. Injectable IOLs use the capsular bag to form the shape of the lens. Provided the elasticity of the refilling material is sufficiently low, the lens shape can then be manipulated by the ciliary muscles and zonules as occurs with the natural lens. Consequently, such injectable IOLs are able to accommodate in vivo. By replacing the hardened lens material of a presbyopic patient with a soft gel injectable IOL the patient's ability to change focus, or accommodate, can be restored.
Apart from problems with in situ curing, such as controlling the crosslinking process and finding clinically acceptable conditions, there has been a struggle to develop polyorganosiloxane compositions for use as injectable IOLs. Injectable IOL materials need to have a suitable viscosity for injection, a suitable refractive index, suitable mechanical characteristics after curing, i.e. modulus, good transparency, be biocompatible, including having minimal extractables and be sterilisable.
The properties, such as viscosity, modulus and extractables, differ from those required for deformable IOLs. Consequently, materials useful in deformable IOLs are by no means suitable for use as injectable IOLs. For example, polydimethylsiloxane (PDMS) has been employed as a material in foldable or deformable IOLs. In the injectable IOL context though, PDMS has been found to have a relatively low viscosity and thereby a tendency to leak out of the injection site (i.e. the capsular bag) before curing. To address this deficiency, high viscosity polysiloxanes have been added to the PDMS reaction mix. However, a drawback of high viscosity silicones is that they can entrap air bubbles, which can impair the optical quality of the resulting product. Also, they are difficult for a surgeon physically to inject in a very delicate environment, often requiring substantial force. In addition, it has been found that polyorganosiloxanes having a high fraction of dimethylsiloxane units may have an unacceptably low specific gravity with the undesired result that the injected lens material will float on any aqueous layer present in the capsular bag. In such a case, it will be difficult to fill the capsular bag completely and will require the surgeon to manually express intra-capsular water in order to maintain the correct lens shape during the filling and curing process.
Therefore, it is desirable to formulate processes for replacing the natural lens with an accommodating IOL that provides optimal results in vivo. Further, it is desirable to formulate injectable lens forming materials from polysiloxanes that has a suitable refractive index and the desired mechanical and optical qualities so as to constitute an optimal replacement for the natural lens.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.
As used herein, the term “comprise” and variations of the term, such as “comprising”, “comprises” and “comprised”, are not intended to exclude other additives, components, integers or steps.